A bill set to pass Virginia's legislature this week risks criminalising doctors by obliging them to carry out an invasive procedure that could constitute a sex crime under state law, Democratic representatives have warned their Republican counterparts.

The ultrasound bill, one of two of the most restrictive anti-abortion measures heard in the state in recent years, could force women undergoing first trimester abortions to submit to a vaginally invasive procedure, offer them images of the foetus and have the resulting image lie on their medical file for seven years.

Democratic delegates have so far fought in vain against the controversial bill in Virginia, which has a Republican majority.

In a dramatic change of tack, they now hope to persuade Republicans that the bill is untenable as it currently stands because it will criminalise doctors under a local statute known as object sexual penetration (OSP), which carries a five-year jail sentence.

They are hoping Republicans will accept an amendment into the bill that requires a woman's consent for the procedure.

David Englin, a Democratic delegate, said: "Object sexual penetration is a serious sex crime in Virginia. It is very difficult to look at the bill and look at the OSP statute together and think that you are not asking doctors to commit a sex crime."

Critics of the measure, which is opposed by 55% of the state's residents according to a recent poll, have described it as emotional blackmail and say it is part of an attempt by the state to bully women seeking abortions to continue with their pregnancies.

Englin and fellow Democrats hope to make what they describe as a heinous measure "less bad" by introducing consent.

"Consent is a key element in the criminal statute, and there is no consent required in the ultrasound statute," said Englin.

Englin said he was particularly disturbed by the prospect of women made pregnant by rape who had gone to their doctor seeking a pharmaceutical abortion to be forced to undergo an invasive procedure.

Charniele Herring, another Democrat, said: "What's before us is akin to rape. The bill requires a transvaginal ultrasound and for that to happen without any appropriate medical purpose. The OSP that says if you insert something in somebody without their consent or without a bona fide medical purpose it is a criminal act. The proponents of the bill have not come up with a legitimate medical purpose."

The bill has sparked outrage among women's rights advocates. Three petitions against the measure, one addressed directly to Governor Bob McDonnell, have already garnered 25,000 signatures.

On Monday hundreds of people lined the streets outside the state's capitol building, protesting against the ultrasound measure and another bill which would declare that personhood began at the point of fertilisation, giving foetuses the same rights as people. The bill, which was due to be heard on Monday, was passed by but is due up again this week and could be passed as early as Tuesday.

The bills have made Virginia the new front line in the battle over abortion. Anti-abortion activists increasingly focus on state legislation to chip away at abortion rights granted by the historic Roe v Wade decision by the US supreme court, following failed attempts to introduce personhood laws through popular ballots in Mississippi and Colorado.

Herring said she is also concerned about the effect of the Personhood Bill, which, if adopted, would make Virginia the first state to define a person at the moment of conception. It would outlaw FDA-approved contraception in the state, she said.

If the ultrasound bill is adopted, Virginia will become the eighth state to require ultrasounds before abortions. Along with a push for personhood, new ultrasound laws have become a key tactic of the anti-choice movement.

Two other states, Oklahoma and North Carolina, have similar rules which are currently being held up by legal challenges. In Texas and Oklahoma, where the rules go further than those proposed in Virginia, a doctor is required to present the ultrasound image of the foetus to the woman seeking an abortion.

Dr Jen Gunter, a gynaecologist and blogger based in northern California, said that she opposed all ultrasound laws because there is no medical evidence to support them and because she was against the "intrusion of politics into medicine".

In her blog looking at the medical evidence behind ultrasound laws, she cites a 2009 study which looked at whether viewing an ultrasound was something women wanted and whether it had an impact on her choice to have the procedure. It concluded that, when given the option, 73% of women chose to view the image and 85% of them felt it a positive experience. Not one woman changed her mind about having the abortion.

Gunter said that ultrasound laws were "a waste of taxpayer dollars" as they do nothing to forward the goal of reducing abortion. They also set a "dangerous precedent of allowing hypocritical politicians to set unacceptably low standards of medical care based on political goals, religion, and misogyny" she said.

Gunter said: "What if a new study is published next week, saying we have been wrong about ultrasounds. Are we then going to change the law? There's no other area of medicine where politics steps in."

Pro-choice campaigners said that the bill, which would introduce a 24-hour wait between the procedure and the abortion, could also add to the cost.

Campaigners say that the Virginia bills are part of an increasingly hostile legislative landscape to women's reproductive rights across the country.

Virginia is one of 19 "anti-choice" states, according to Naral Pro-Choice America, where they have anti-choice legislatures and an anti-choice governor.

"In places like Virginia women don't have a firewall to protect their reproductive rights," said Ted Miller of Naral. "We've been tracking these kind of laws since 1995. In 2011 there were 69 anti-choice measures across the country – double the number there were in 2010."

He added: "There is a hostility to women's reproductive rights across the board. 2011 was a very bad year and prospects for 2012 are equally bad."

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